This invention relates generally to gas turbine engines and more particularly to exhaust nozzles having noise reduction features. In gas turbine engines, thrust is produced by imparting a large acceleration to a small mass of high-energy gas, thereby creating forward momentum of the engine. This is chiefly accomplished by combusting fuel in a stream of compressed core air, which is then passed through one or more turbines and out an exhaust nozzle. One turbine typically drives a compressor for producing the high-energy gas. The exhaust nozzle further increases the acceleration of the gas. A second turbine may also be used to power a turbofan for imparting acceleration to a secondary stream of air, typically concentric with the core airflow, that is also passed through an exhaust nozzle. The two streams of air mix with each other and with ambient air streams as they exit the engine from their respective exhaust nozzles.
A primary source of jet engine noise is the shear region of the exhausted air streams, where different high-velocity air streams mix with each other and the slower moving ambient air. Especially in turbofan engines, where the secondary air stream exits the engine at approximately 1000 ft/sec and the core air stream exits the engine at approximately 1600 ft/sec, the mixing of the different velocity air streams produces a great deal of turbulence and associated noise, particularly low-frequency noise.
Due, in part, to standards developed by various air transportation industries and government bodies, efforts have been made to reduce the overall noise production of aircraft for various purposes including improving quality of life near airports. The noise produced at the exhaust nozzles has long been known to be a major source of aircraft noise, and various attempts have been made to reduce exhaust gas noise, particularly the low-frequency noise. For example, various noise reduction systems employ tabs along the downstream perimeter of exhaust nozzles to produce smoother mixing of the various velocity air streams, thereby reducing the levels of noisy shear flows. These systems, however, achieve low-frequency noise reduction at a penalty to high-frequency noise and thrust production. Therefore, it would be desirable to have an exhaust nozzle with improved noise reduction characteristics, while also reducing associated thrust loses.